5. Field of the Invention
The present invention relates to a data transmission system for data transmission among plural stations, and more particularly to a data transmission system adapted for use in a local area network.
2. Description of the Prior Art
Recent progress in office automation has stimulated the development of a communication network in a limited area suitable for data transmission among various office equipment, which is usually called a local area network (LAN). In a conventional local area network the data transfer rate and data format in the transmission channel are fixed. Therefore, in the case of connecting an unmatched terminal to the network, it becomes necessary to provide matching means such as a data buffer for matching with the transfer rate of the network or means for converting the data format, and such requirements lead to a more complicated structure and increased cost.
In order to avoid the above-mentioned drawbacks the present applicant already disclosed, in a patent application Ser. No. 521,831 for a Data Communication Method, filed Aug. 10, 1983 by the present inventor, a communication network system in which each station can transmit data having an arbitrary format with an arbitrary transfer rate, without any hardware for transfer rate matching or for data format conversion. More specifically, in the system of said patent application, data transmitted through the network are classified into two categories; one being protocol data of a fixed data format and of a base rate at which the protocol functions, and the other being special data for data transmission which are of an undefined data format and of an arbitrary rate to be determined by the ability of the terminal. The transmission of said special data through the transmission channel is always preceded by a communication control command by said protocol data which informs all the nodes or stations in the entire network of such transmission.
The above-identified patent application describes a process in the stationary state of a so-called network in which stations and network are both ready for communication and control by protocol is effective. In the actual network system, however, in addition to the above-described process in the stationary state, there is required a so-called synchronizing process for activating a station and synchronizing the same with the communication sequence already in progress in the network, thus realizing the above-mentioned stationary state, and there is usually required means for this purpose separate from the network control protocol.
In the conventional network, the transmitted data has a predetermined data format and a transfer rate similar to the aforementioned protocol data, in which special flag codes or synchronization signals indicate the start and end of the effective data in the flag areas in front of and behind the data as in the high-level data link format (HDLC) shown in FIG. 1. In the conventional network, for the purpose of achieving the synchronization to the communication sequence, an exclusive discriminating circuit is utilized to discriminate these identification codes at a determined frequency, thereby detecting the junctions between the transmitted data and enabling the reception or transmission of data after said detection.
However, such a conventional synchronizing method cannot be merely applied to the network system in which, as described in the aforementioned patent application, data of different speeds and different formats (quantities, modulating methods etc.) such as image data are mixed, since such data of different speeds and different formats may contain codes or signals which are the same as or are mistaken by the discriminating circuit as the identification codes.
Such a local area network employs only one communication line for cost reduction, and the characterization of the data in said communication line is achieved not by a special auxiliary signal line but by so-called protocol.
Protocol in communication means a common understanding among all the stations in the network according to which all the stations interpret the transmitted data, and, in this manner the transmitted data composed of a group of bits can have definite meaning.
In the following there will be given an explanation on the conventional mechanism of data transmission and reception.
In most cases, the data transmitted in the local area network is associated with a receiver address code and a transmitter address code.
The receiver address code is used for determining the receiving station of the transmitted data.
More specifically, the data transmitted to the communication channel are equally received by all the stations, except the transmitting station, in the network, and the receiver address code is checked. The data are accepted by a station whose station address coincides with said receiver address.
On the other hand, the transmitter address represents the address of the transmitting station, and is utilized for making a response from the receiving station of the transmitted data, but the frequency of utilization is apparently less than that of the receiver address.
In the conventional system, each data are associated with only one receiver address, and the communication is made in so-called one-to-one type in which data can be transferred to only one station in one communication.
Consequently, a complicated control procedure is required in one-to-plural type communication.